This is probably an ignorant question, but can I ask why you couldn't just take a 690 frame and drop in an RFS engine? Or just use an RFS bike and strengthen the frame and subframe?

Hi Big Mike! - not an ignorant question at all, but as Barrier has just reiterated above, there are a number of additional benefits to retaining the basic 690 bottom end, and of course the 690 engine is EFi too.

That said, it is certainly another option to drop in a different [Enduro based] engine - basically what the Factory did with the 450RR... and the older (RFS design you refer to) engines are one of the few that might well go the distance... unfortunately they are very hard to come by now (and are all of course secondhand), while the newer XC4 design engine is even more highly strung and has had issues of its own regarding oil circulation etc.

Ultimately, we feel the 690 Enduro (with the addition of the Rally Raid Evo2 tank and faring kit of course) is a solid and dependable 'open class' rally bike for the privateer... what we are doing is making it conform to the Dakar 450cc regulations with hopefully the minimum of changes - allowing us to showcase it on the world stage, and prove it's reliability in what is undoubtably ultimate endurance event. If it is successful, then the engineering offers other riders (and owners of older 690RRs for example) the option to campaign a similar machine in the future.

Hi Big Mike! - not an ignorant question at all, but as Barrier has just reiterated above, there are a number of additional benefits to retaining the basic 690 bottom end, and of course the 690 engine is EFi too.

That said, it is certainly another option to drop in a different [Enduro based] engine - basically what the Factory did with the 450RR... and the older (RFS design you refer to) engines are one of the few that might well go the distance... unfortunately they are very hard to come by now (and are all of course secondhand), while the newer XC4 design engine is even more highly strung and has had issues of its own regarding oil circulation etc.

Ultimately, we feel the 690 Enduro (with the addition of the Rally Raid Evo2 tank and faring kit of course) is a solid and dependable 'open class' rally bike for the privateer... what we are doing is making it conform to the Dakar 450cc regulations with hopefully the minimum of changes - allowing us to showcase it on the world stage, and prove it's reliability in what is undoubtably ultimate endurance event. If it is successful, then the engineering offers other riders (and owners of older 690RRs for example) the option to campaign a similar machine in the future.

And yes, perhaps we're just a little bit crazy too!

Jx

Don't know about crazy, I think it's an awesome idea... Careful of checking, double checking and checking again on the scrutineering part for the Dakar, I seem to recall a whole bunch of modified quads (Macachek included) being disqualified at the last minute for Dakar '12... that would suck big time, not trying to be a nay-sayer, just pointing out the sometimes fickle ways of the organizers

This is certainly a platform I would be looking into for the future, maybe not for the Rally Lite races around my neck of the woods, where all you need is an enduro bike with a larger tank + basic nav equipment (think Sardegna), but seeing as the Africa bug has bit hard, I am looking into some races like the Africa Eco Race (where you can still race a 690) or the Tuareg... this looks to be awesome, and I don't just mean the engine conversion but the whole Rally Raid UK Evo2 kit.

I don't have much to add, other than I'm stoked that we'll get to check in on the project in person at the Tuareg.

I was initially skeptical about the project, but as you've taken the time to explain the project I have grown increasingly interested and impressed. This is no simple re-sleeve or parts swap! I think it's a pretty great idea.

As John explains above, the combination of a longer rod and adjusting the position of the crank pin will give the necessary stroke to retain the standard bore/cylinder etc. As he explains,the trick[y bit] will be to create a piston profile that creates enough compression while at the same time misses the valves when they are extended - the 690 Rally cam (which should help boost power) has a longer overlap than the stock cam, so things could be a bit tight in there - hence the prototype build and bench test before we go to production with the final specification...

Having wasted my youth on law school and financial management, I would like a little more "schooling" from the folks on the thread-

in order to get to 450 cc. displacement with the same bore, the stroke has to be reduced to 55mm. Moving the crankpin 12.5 mm closer to the crankshaft center achieves the stroke reduction. However, this also drops the the top of the piston below the cylinder deck height at TDC, producing an unacceptable loss of compression.

How does a longer connecting rod fit into the picture? If the rod is lengthened to move the top of the piston to the necessary height (and proper compression), doesn't that also decrease the displacement (piston higher at BDC)?

It's probably obvious to everyone but me - still, a little more education will help.

How does a longer connecting rod fit into the picture? If the rod is lengthened to move the top of the piston to the necessary height (and proper compression), doesn't that also decrease the displacement (piston higher at BDC)?

It's probably obvious to everyone but me - still, a little more education will help.

Thanks

Greg

Stroke is determined by the diameter of the circle travelled by the bottom end of the connecting rod. The closer the big end is to the center axis of the crank, the lower the stroke. If you reduce the radius of the the circle travelled by the big end you need a longer connecting rod to get the piston back up where it needs to be. Just draw a picture of a crank, connecting rod and piston at TDC. Then imagine moving the lower end of the connecting rod closer to the center axis of the crank while keeping the piston in the same place.

Stroke is determined by the diameter of the circle travelled by the bottom end of the connecting rod. The closer the big end is to the center axis of the crank, the lower the stroke. If you reduce the radius of the the circle travelled by the big end you need a longer connecting rod to get the piston back up where it needs to be.

In some circumstances you could but not likely with this engine. With the smaller stroke you need to run the top of piston a little higher at TDC to maintain the same compression ratio. (You are only squishing 450 cc into the space above the piston where you used to squish 650 cc.) That makes it closer to the lower end of the valves when they are open (the end of the exhaust stroke). You need to reshape the top of the piston to prevent contact with the valves and there could also be issues with ring placement depending on the particular engine cylinder/head design. The nice thing then is you have a piston that could be placed in the standard stroke engine that would raise the compression ratio over stock. Cool idea.

Top explanation there Sparrowhawk! - I don't think either John or myself could have said it any better!

As you say, to raise the compression ratio back to where we would need it for reasonably decent performance, the piston in effect needs to move a little closer to the top of the cylinder/combustion chamber... we are hoping that by essentially doming the top/centre of the piston, it will create the necessary compression, while still allowing clearance for the valves, and at the same time not buggering up the whole efficiency of the chamber itself... (suck squeeze bang blow).

The only other thing I would add to give Greg (or anyone else) a clearer picture of the mechanics of the conversion is that in using a longer rod with a shorter stroke but in the original cylinder/barrel - we are effectively no longer using the lower part of the cylinder wall that was swept with the stock crank and rod...

Top explanation there Sparrowhawk! - I don't think either John or myself could have said it any better!

Jx

A big Thanks to all of you. Sparrowhawk - I like your idea of drawing a picture - "See Spot run" always worked for me!
Any idea where I could find some of the engine's measurements - rod length (center-to-center), piston (crown-to center-of-wristpint),crankpin-to-center-of-crankshaft?

Hee hee - funny you should say that!
Certainly the plan this year is for me to ride the 450 to the Hellas Rally and back - that is from the UK down to Italy to get the ferry, race the event, and then ride it home again - to really prove the point that this engine is going to be ultra reliable!

Nice... looking forward to see how it will perform at the Hellas Rally... see you in Greece!